
I had a dog like this. She's now fine. You need to NEVER force your dog to meet another, never try to be "in charge", rather give her the opportunity to hide behind your legs or walk away, not having to get too close. If you were scared of snakes, putting you in a room full of snakes wouldn't help -it would only make things worse. It's the same with dogs. :) She needs to learn that she doesn't have to be afraid, because nothing bad will happen again. She tries to bite other dogs because she thinks that the best option is to attack first before she is attacked herself. It's a long and slow process, but it DOES work. You do need to have her out on her own. At first you go nowhere near other dogs, only stay as far away so that she can see them without being worried and without reacting. When she sees another dog and does nothing, even if it is far in the distance, lots of praise and treats. Gradually move closer over a period of time -this can take weeks. Never punish, never tell off if she has a go at another dog, just quietly walk away in the opposite direction. It would help greatly if you had a friend with a calm and reliable dog that you could practice with, and just move closer and closer to, again over a period of time. You don't even have to walk to start with, sitting down somewhere and doing nothing is fine. It's important that she sees that you ignore other dogs and are not worried in any way.
My Ginny was exactly like this when I got her. (She had the other issues you describe as well, like ducking down etc.) She would attack any big dog she saw (and she is just a Papillon), out of fear. She terrorised my mother in law's Labrador so badly she became terrified of Ginny. Ginny would run at her from behind and bite her. We did all the above. It took months, but today I can take Ginny to a training class and have her in a hall full of other dogs, and she doesn't even bark at them. It's nearly a year on now, and if she gets worried I allow her to back away, I never force a meeting, so she knows that she doesn't have to defend herself as I make sure nothing will happen to her. I was lucky that my MIL's Labrador was a calm dog that would never retaliate, and I was able to get my step son to help me -he would have the Labrador on a lead and I would have Ginny and we would just work through everything over time. Sometimes all it meant was sitting down in a chair each with the dogs on a lead, far from each other, not doing anything. Then moving closer and closer, and when Ginny kept calm, rewarding her a lot.
It's vital that you don't tell her off, force her or try to be firm as this will only make things a lot worse with a nervous dog.